This invention relates generally to an improved hoist. More particularly, but not by way of limitation, this invention relates to an improved hoist having a direct current brake and to the control provided therefore.
Various arrangements of both alternating current and direct current hoist brakes have been utilized with success in the past. It is highly desirable to cause the brakes to actuate as rapidly as possible, and, of course, to utilize as little energy as possible.
U.S. Pat. No. 3,830,344 which issued Aug. 20, 1974 to Cervenec et al. and No. 4,232,768 which issued Nov. 11, 1980 to Jacques Dufresne illustrate brake systems for hoists that are spring actuated and deactivated by means of solenoids. The patents illustrate braking systems for hoists, but do not illustrate the energy saving system of this invention.
Looking at a broader field, that is, the use of solenoids for various other purposes, two patents have been located, namely U.S. Pat. No. 3,761,730 which issued Sept. 25, 1973 to Thomas A. Wright, and U.S. Pat. No. 4,114,184 which issued Sept. 12, 1978 to Harald Stampfli illustrate the use of a reduction in power to maintain the solenoid plunger in position after the large initial moving force has been exerted. In the systems illustrated in the patents, the solenoids are controlled either by a multiple coil or by utilizing a center tap on a single coil. In the foregoing systems, only a portion of the coil or a single coil of two coils is used to retain the plunger in its actuated position. Thus, a reduction in the amount of power utilized is attained compared to applying full power to actuate and retain the solenoid plunger.
It is an object of this invention to provide an improved hoist wherein the brake system is controlled so as to reduce the power required to maintain the brakes in the unactivated position, and thus conserve a substantial amount of energy while at the same time, providing a brake that can be quickly activated.